1. Field of the Invention
The present invention is related to a fixing unit for fixing a toner image to a sheet and an image forming apparatus with the built-in fixing unit.
2. Description of the Related Art
An image forming apparatus such as a copier, a facsimile machine or a printer typically includes a fixing unit configured to fix a toner image to a sheet. A certain type of the fixing unit includes a heating roller and a pressure roller configured to press the heating roller. While a sheet bearing a toner image passes through a nip portion between the heating and pressure rollers, the toner image is fixed to the sheet.
Another type of the fixing unit includes a heated belt and a pressure roller configured to press the belt. While a sheet passes through a nip portion between the belt and the pressure roller, a toner image is fixed to the sheet.
The aforementioned belt-type fixing unit includes a heating roller on which the belt is wound and a fixing roller which nips the belt in cooperation with the pressure roller, in addition to the belt and the pressure roller, which are described above. The belt is wound on the heating and fixing rollers. The fixing unit typically includes the heating roller formed with bushes for stabilizing the belt tracking. The bushes held in contact with the lateral edges of the belt prevent the meander of the belt.
The meander of the belt (variation in tracking position of the belt in a lateral direction) adversely affects quality of a toner image to be fixed and conveyance of a sheet after a fixing process. A frame supporting the pressure roller and a frame supporting the heating and fixing rollers on which the belt is wound are typically separately provided. Errors in assembly between these frames and machining errors of both frames affect the meander of the belt. Accumulation of these error factors causes the meander of the belt which the aforementioned bushes may not overcome.